The invention relates to an electronic printed circuit board which is designed for a display instrument, particularly in a motor vehicle, and has a light source.
Display instruments for modern vehicles have a printed circuit board holding the essential components. In addition to the necessary electronic components, this printed circuit board usually also accommodates a pointer drive system for an analog pointer-type instrument, a light source and possibly other mechanical and/or optical components. The thus populated printed circuit board can then be easily inserted into an appropriate housing of the display instrument and can then be fitted, together with this display instrument, into a dashboard in a motor vehicle. To illuminate a dial plate and for background or transmitted illumination of symbols, warning indicators or monitoring displays, the printed circuit board is fitted with at least one light source, which is often connected to an optical fiber. In particular, the light source is also used for actively illuminating a liquid crystal display (LCD).
A disadvantage of printed circuit boards of the aforementioned type is that any change in the position of the light source on the printed circuit board usually requires that the printed circuit board be completely reconstructed. This means that the use of the printed circuit board is generally restricted to a single application, and that changing the display instrument is associated with considerable complexity.
It is already known practice to connect the light source to an optical fiber so that the area which is to be illuminated can then be reached better. In this arrangement, a diffuser ensures that light emerges uniformly at the desired point, the optical fiber itself being of very shallow design and affecting the physical height only insignificantly. A disadvantage of this is that the light emerging from the diffuser, particularly for illuminating large areas, such as are found in actively illuminated liquid crystal displays, does not allow a sufficient illumination level. There are then frequently differences in brightness, which the observer perceives as irritating and which make reading more difficult. In addition, the difference in brightness gives an inferior impression which cannot be prevented even by using more powerful light sources.
The invention is based on the problem of providing a printed circuit board of the aforementioned type which is intended for a display instrument in such a way that it can be used flexibly for different display instruments and the illumination level is increased at the same time. In particular, it should also be possible to achieve particularly uniform illumination without differences in brightness.
The invention solves this problem in that the light source is arranged on a support which is designed as a bridge and is mounted on the printed circuit board by a fixing means. This allows the area under the support to be used for other electronic components, which means that the design can be compact. The support can be mounted at no distance from the populated printed circuit board, so that there is only an insignificant increase in the physical height, or none at all, as a result. At the same time, the support itself can be made of a comparatively thin material, since it does not need to have any conductor tracks. In addition, the arrangement of the light source on the support allows flexible positioning of the light source on the printed circuit board, without any change in the electronic components or rearrangement of the conductor tracks being necessary for this. To this end, the support can easily be positioned at a predetermined point using the fixing means, which allows the fixing, which can be both detachable and undetachable, to be immediately monitored visually and mechanically. This allows flexible adaptation to different applications, and, in particular, further supports can be added and easily replaced for servicing. In addition, the power loss from the light source in the form of heat has no disadvantageous effect on electronic components arranged on the printed circuit board, since the physical decoupling provides for improved heat dissipation. Electrical contact with the light source could be made by means of cables, for example; however, it is particularly beneficial if the support is designed as a rigid or flexible printed circuit board and holds conductor tracks for supplying the light source with electrical power. The extraordinary compactness means that the printed circuit board according to the invention can also advantageously be used, in particular, for combined display instruments having a plurality of displays, as used in motor vehicles, in particular.
A particularly advantageous embodiment of the invention is provided when the fixing means has a clip connection. This allows easy connection of the support to the printed circuit board without additional auxiliary means or tools. It also allows easy implementation of a practical modular design, so that an individual display instrument is produced by connecting individual components using the clip connection.
In this regard, a development of the invention which is particularly beneficial is one in which the fixing means is integrally connected to a housing of the display instrument. This allows the support to be connected directly to the housing, which means that there is no need for complicated positioning, and the number of components is reduced. In this case, the printed circuit board connected to the support is secured in the predetermined installation position subsequently, for example as an addition.
It is also beneficial if, according to one development of the invention, the fixing means is a hinge. This allows the printed circuit board and the support to be premounted, first, irrespective of the subsequent installation position, enabling the support to be pivoted into the final position after the printed circuit board has been fixed. This means that, in particular, configurations of the type in which the display plane and the plane of the printed circuit boards are not parallel to one another can also be produced.
Assembly is particularly simple if the fixing means has an electrical contact arrangement. In this case, electrical contact is made at the same time as the support is fixed, in only one operation. Furthermore, possible sources of error both in assembly and in operation can then be largely eliminated.
In this case, the support is fixed particularly simply in that the support is bonded to the printed circuit board. This allows the support to be arranged at virtually any desired point on the printed circuit board. This requires no special preparation or configuration of the printed circuit board. At the same time, an adhesive or a double-sided adhesive strip used for bonding also serves for insulation and protection of the printed circuit board against mechanical damage.
Electrical contact could be achieved by means of a solder connection. On the other hand, a particularly advantageous embodiment of the invention is provided in that the support can be placed in electrical contact with the printed circuit board by means of a plug connection. Such a plug connection requires only a small degree of complexity in the assembly process, and can easily also be retrofitted at a later instant, for example if further displays are desirable. At the same time, the plug connection also allows the use of a support which is not suitable for a solder connection on account of the thermal or mechanical loading, or which, to simplify the assembly process, cannot be connected to the printed circuit board until at a later instant.
By way of example, the support could be hardboard or rigid-plastic board. According to another development of the invention, however, it is particularly advantageous if the support is a ceramic substrate. This ensures good dissipation of the heat given off by the light source. Conductor tracks can be printed on the substrate for making electrical contact with the light source.
One embodiment is particularly advantageous if the support is a foil. This firstly produces a comparatively small thickness, which results in a very compact design for the display, and secondly the foil allows the light source to be adapted flexibly to different installation positions. In particular, this also allows curved surfaces at different levels in height to be produced.
In such a case, it is particularly beneficial if the support is elastically deformable. This means that, in the installation position, the support can be arranged such that it is prestressed against the printed circuit board, which makes simple, reliable fixing possible. At the same time, the support comes to bear optimally against the printed circuit board, so that a compact design can be achieved. This likewise allows for simpler connection of a support having the light source and the contact arrangement provided only on the front, because, first, the surfaces of the support and of the printed circuit board are placed onto one another and contact is made with them, and then the support is folded over, so that the light source subsequently points in the direction facing away from the printed circuit board. Furthermore, any components of the printed circuit board which might be arranged underneath the support can be accessed without any difficulty, even for servicing, by lifting off the support from the printed circuit board.
In this context, a development of the invention which is also particularly simple is one in which the support can be rolled. This means that it can be produced economically by the meter with a light source which is already fitted, for example, and that it need merely be cut to the desired length in the assembly process.
In this context, it is also particularly advantageous if the support has a reflector for the light source. This allows the desired area to be illuminated specifically and uniformly with little complexity. For example, the support can also be designed such that the light source inserted in the reflector does not protrude from said reflector, which means that the light source is arranged so as to be protected against damage.
The support can be made of any desired material. However, one embodiment of the invention is particularly beneficial if the support comprises a section of the printed circuit board. The section of the printed circuit board, together with the printed circuit board, can have components fitted in a single operation. When the section has been separated off, it forms the support. This allows the manufacturing process to be simplified and means that, by cleverly utilizing the geometry of the printed circuit board, an otherwise useless residual portion (waste) can be used as a support.
The light source can be a conventional incandescent lamp, for example. A particularly advantageous embodiment of the invention is one in which the light source is a light-emitting diode (LED). Such light-emitting diodes can be soldered directly to the support and can be arranged next to one another with only a small gap, so that the result is a uniform light source with little susceptibility to faults. In this arrangement, such light-emitting diodes afford the possibility of radiating light of different colors, depending on the driving.
It is also particularly beneficial if the support is an electroluminescent foil. This electroluminescent foil inherently combines the function of the support and of the light source, so that further components can be dispensed with and consequently a significantly smaller physical height can be achieved.
A preferential improvement in the heat dissipation from the light source can be achieved, and it can also advantageously become possible to arrange additional components on the printed circuit board in the area of the support, if, in accordance with another development of the invention, the support does not rest directly on the printed circuit board, but is at a distance from it.
Particularly if the light source has a high power loss, it is especially advantageous, according to another development of the invention, for a heat-dissipating element to be arranged between the printed circuit board and the support. The heat-dissipating element can be a passive cooling element, for example in the form of metal cooling ribs, or an active cooling element, for example in the form of a cooling line charged with coolant.
It is conceivable for the support to be arranged approximately parallel to the printed circuit board; however, in accordance with another advantageous development of the invention, in which the support is arranged at an acute angle to the printed circuit board, the light source can be oriented specifically onto the area which is to be illuminated, which increases the illumination level.